With the development of mobile communication technology, people are no longer satisfied with the simple speech communication fashion. Instead, they need a brand new multimedia communication fashion, which includes not only simple audio/video communication, but also instant messaging, browsing at-one-site, cooperative work, and stream media services, etc., especially services combining new communication approaches with conventional speech communication. Based on the trend of combination between Internet and telecom networks, various standardization organizations have put forward the concept of next generation network (NGN). 3GPP has introduced IP multimedia subsystem (IMS) in full IP service network architecture based on packet-switched bearer network, with the aim of masking off the user access modes, controlling the degree of openness of service capability, and providing multimedia communication experience according to personalized user data. The main functional entities in IMS include Call State Control Function (CSCF) entity that controls user registration and session control, etc., Application Server (AS) that provides various service logic control functions, Home Subscriber Server (HSS) that provides central management of subscribers' subscription data, and Media Gateway Control Function/IMS Media Gateway (MGCF/IM-MGW) for implementing intercommunication with circuit switched (CS) networks. Users access IMS via the proxy CSCF nodes (P-CSCFs) in the networks in which the users are currently located, while service control as well as interaction with AS are accomplished by the home serving CSCF nodes (S-CSCFs); IMS employs Session Initiation Protocol (SIP) for signaling exchange and session negotiation, and as the interface protocols between terminals and CSCF, between CSCFs, and between CSCFs and AS.
A conventional communication network comprises circuit switched (CS) domain and packet switched (PS) domain, which accomplish CS service (including voice service, circuit-type data service, and fax service, etc.) and packet-based data service, respectively. IMS service comprises control part and media content part; wherein, the control part is IMS signaling exchange between IMS terminals and network entities in IMS domain to control IMS service; the media content part comprises various types of media including image, speech, data, audio, etc. Both IMS signaling and media streams are carried on PS domain. Because CS network and PS network use network resources differently (CS network monopolizes resources, while PS network shares resources), for services having higher requirement for time delay, such as voice service, the Quality of Service (QoS) will be degraded by a certain degree if the service is implemented in IMS, compared with that implemented in CS, thus it is hard to meet the service requirements of the operators and subscribers. In view of this, the International Standardization Organization 3GPP has put forward Combining CS Bearers with IMS (CSI) services, which comprises two implementations: CS/CSB and IMS/CSB; wherein, CS/CSB refers to CS/IMS parallel integrated services, i.e., CS calls and IMS sessions are fully controlled by corresponding domains respectively, and correlated at the UEs of both parties, thereby providing richer services taking the advantages of CS/IMS integration; IMS/CSB refers to CS bearer being used in IMS sessions, that is, the establishment of CS bearer which is specifically performed in CS domain is controlled by IMS service logic, so as to solve the Quality of Service (QoS) problem in the part of IMS Services that have higher requirement for time delay.
Since the implementation of IMS/CSB requires cooperation of UEs and IMS network, and is relatively more complex, 3GPP decides to divide CSI into two phases so as to implement above services as soon as possible: in phase 1, end-to-end CS/CSB shall be implemented on the basis of the capability of presently existing networks as far as possible. CS calls and IMS sessions are controlled respectively in CS domain and IMS domain. IMS sessions can be added over established CS calls, or CS calls can be added over established IMS sessions, with correlation between CS calls and IMS sessions being only accomplished at the UEs of both parties according to the requirements of the specific service application, as shown in FIG. 1.
Since the above-described services in CSI phase 1 require the UEs and the networks at both parties to maintain air interface connection of CS and PS at the same time, end-to-end capability information exchange needs to be implemented before a session/call in the other domain is added over an established call/session; the content of the exchange includes UE capability, capability of the radio access network which is accessed currently, and user identity (E.164 number in CS domain or SIP-URL in IMS domain) for call/session establishment in another domain; in addition, since the CS calls and IMS sessions in the services in CSI phase 1 are controlled by CS and IMS networks respectively, the existing services in the two domains may have adverse effects on the calls/sessions; it is necessary to take into account as to how to guarantee the establishment of calls/sessions between the same two UEs.
3GPP TS 23.279 is the technical standard in 3GPP for services in the CSI phase 1. In the latest version, the end-to-end capability exchange solution provided and proposed for using in the CSI phase 1 is as follows:
The static information, such as the UE capability of the opposite party, is exchanged through SIP OPTION operation on IMS and buffered locally;
In the case that a CS call has been established first, dynamic information, such as the radio capability of network, is exchanged by IE subaddress or UUS 1 (User-to-User Signaling 1) in the call control signaling during the call establishment; and in the case that an IMS session has been established first, it is exchanged through SDP offer/answer mechanism and corresponding extension of SIP header/SDP body (Session Description Protocol body) during the IMS session establishment.
In case that the local party has not stored the UE capability of opposite party, the UE information can also be exchanged directly during the IMS session establishment through SDP offer/answer mechanism and the corresponding extension of SIP header/SDP body.
However, the above described capability exchange method doesn't take into account the case that the same IP multimedia public user identity (IMPU) may be shared by multiple user equipments. For this case, in IMS system, it is handled by serial or parallel Forking mode, that is, for multiple addresses (corresponding to a registered UE sharing this IMPU respectively) under this IMPU, the system forwards service requests to them sequentially or in parallel, at this time, it is impossible to determine in advance which UE will respond to this service request ultimately. Therefore, as OPTION operation can only be initiated for IMPU, in the case that one IMPU is shared by multiple UEs, the UE of the opposite party is unable to determine the capability of which UE has been exchanged and stored, and to know whether or not the UE which established the session during the subsequent INVITE procedure is the certain UE whose capability has been stored at the local party.
Furthermore, the user may change the terminal in use at any time, and the capability of each terminal may be different. The current capability exchange mechanism has no means which can detect effectively that the user of opposite party has changed terminal and re-exchange of capability information is needed. This makes the UE capability information of the opposite party stored at local party lose reliability. In addition, periodically updating of capability information blindly can not solve the problem effectively and will cause extra burden on the network.
In addition, the services in CSI phase 1 require the CS call and IMS session that have been established successively must be established between the same two UEs. In the case that a CS call is established first, it is permitted for the service control mechanism, such as call forwarding on CS, to determine the ultimate receiver, and display the user number connected ultimately to the opposite party by call display related services, so that the subsequently initiated IMS session can be established between exactly the two UEs. As there may be the cases that IMPU is shared by multiple UEs in IMS and some services in IMS domain may result in session redirection, though it is possible to know the user number of the opposite party in the established CS call and to obtain the corresponding IMPU according to the existing solution, it is unable to ensure that the IMS session be established as expected to that certain UE of opposite party to which the CS call has been established.